Several agents for impregnation of combustible materials are known. General problems with the known agents in general are: "sweating-out" of the impregnation agent and miscolouring of the impregnated material, a time consuming and expensive impregnation process comprising several steps is required, the impregnation agent is toxic, or that the fire- and fume inhibiting effect is simply not good enough.
More and more focus has been given to the flame inhibition of combustible wood and cellulose containing materials. Flame inhibition of wood materials has been developed in many areas in order to achieve a higher degree of safety for users of wood constructions, and can generally be divided into two main groups:
1. Flame inhibitors and protection methods that allow the wood and cellulose containing materials to keep their fire inhibited properties even after exposure to weather, wind and water. These are denoted "for outdoor use". PA1 2. Fire inhibitors for wood and cellulose containing materials that do not maintain their fire inhibiting effect after being exposed to weather, wind or a relative humidity (RH) beyond approximately 90%. The fire inhibiting agents absorbed will tend to migrate outwards in the substrate material in order to equalize the concentration gradient, whereby the substrate material over some time will lose some of its fire inhibiting effect. PA1 This category of fire retardants is denoted "for indoor use" and represents the dominate type of wood- and cellulose treatment in the present market. This treatment is usually cheaper than treatment with agents "for outdoor use".
In later years there has been a trend in the wood impregnating industry towards use of fire inhibiting products/composition mixtures that give low hygroscopicity and best possible resistance against being washed out and therefore can be used for outdoor purposes. Many of these fire inhibiting composition mixtures have been based on amine-aldehyde-phosphorus condensation compositions. Goldstein et al (U.S. Pat. No. 2,917,408) describes for instance the manufacture of wood with fire inhibiting properties by a combination of dicyandiamide and phosphoric acid, and in U.S. Pat. No. 3,159,503 is disclosed the manufacture of wood with fire inhibiting properties by a combination of dicyandiamide, phosphoric acid and minor amounts of formaldehyde. In addition, Juneja (U.S. Pat. No. 3,832,316) has described a fire inhibiting composition for wood consisting of dicyandiamide, melamine, formaldehyde and phosphoric acid. Juneja also describes (CA patent 917.334) a composition consisting of dicyandiamide, urea, formaldehyde and phosphoric acid to inhibit fire in wood.
Most of the technical solutions mentioned above suffer from one or more disadvantages: formaldehyde is not resistive against being washed out, is not resistant to high humidity, is not storage stable, has too high a hardening temperature (above 100.degree. C.), and is too acidic for the wood, i.e. has too low pH, which over time will cause low compressive strength of the wood and will also cause corrosion of metal parts connected to- or in touch with impregnated wood.
In an attempt to overcome these disadvantages, there has been provided a composition for the impregnation of wood as disclosed in NO patent application No. 953316. According to that method the composition comprises 0.5-40% by weight of monoarnmoniumphosphate and/or diammoniumphosphate, 5-30% by weight of a water soluble and dissociatable ammonium compound, 10-60% by weight of a water soluble metal salt with the ability to form water insoluble salts with phosphate ions and/or ammoniumphosphate ions, 0-30% by weight of phosphoric acid (85%), 1-20% by weight acetic acid, and optionally up to 5% by weight additives, in a total concentration of from 1 to 55% by weight with respect to the total weight of the water solution. With this solution is obtained a high degree of saturation of fire inhibiting agent in the impregnated material with a corresponding good fire inhibiting effect, while at the same time avoiding discoloration of the wood and the known disadvantages usually associated with the use of fire inhibiting compositions as discussed above.
The composition according to NO-A-953316 above has, however, a relatively high content of free acetic acid, which in a process-technical context will be subject to disadvantages: the acetic acid compound will require extra security efforts, such as suitable protective equipment and correctly dimensioned outlet systems with respect to handling and use, and constitutes a potential health hazard for the working personnel. Furthermore, the composition requires a special program for drying plus separate equipment has to be installed to separate the acetic acid from the drying air before being released, and recovered acetic acid has to be treated in separate process equipment before being reused or destroyed. Thus, the acetic acid is a factor that makes the process significantly more expensive and is an environmental disadvantage for the process.
The objective of the present invention is consequently to obtain a method for the manufacture of a fire inhibiting composition for impregnation of normally combustible materials, like wood and other cellulose containing materials and textiles, where the method overcomes the disadvantages associated with known methods. The resulting composition shall further comply with the fire and material technical requirements of the impregnated material.